The issue persists after restarting and resetting HomePod.
The issue occurs intermittently when streaming from a MacBook Pro, and a MacBook both running the latest OSes.
The issue has occurred intermittently since Apple Music replaced iTunes in June 2015. It will go long periods without occurring, and then will occur again and then more frequently, without any apparent pattern.
HDCP errors and problems seem very common out there, but this is especially weird because it's music streaming from Music on macOS to a HomePod, so it shouldn't be involved at all. The reason I control HomePod like this is that Siri is almost totally incapable of playing what you ask for, it almost always plays something else. This way you can directly choose what to play without the game of going back and forth 10 times to try and get Siri to play an album that you are looking at in Apple Music while it says "I'm sorry I can't find anything like that on Apple Music..."
It occurs only when playing music in Apple Music and selecting HomePod as the output. It just occurred again, so I came to see if there had been any response to this from people who have experienced this and figured out why it happens.